The present invention relates to the magnetic resonance imaging arts. It finds particular application in conjunction with black blood magnetic resonance angiography arid may be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with other types of angiography and other types of magnetic resonance imaging.
Measurement of blood flow, in vivo, is important for the functional assessment of the circulatory system. Angiography has become a standard technique for making such functional assessments. Magnetic resonance angiography (MRA) provides detailed angiographic images of the body in a non-invasive manner, without the use of contrast agents or dyes.
Traditionally, MRA methods can be divided into xe2x80x9cwhite bloodxe2x80x9d and xe2x80x9cblack bloodxe2x80x9d techniques. In white blood angiographs or time of flight (TOF) angiographs, magnetic resonance signal from flowing blood is optimized, while signal from stationary blood or tissue is suppressed. This method has been problematic for a number of reasons. First, it is difficult to generate accurate images of the vascular system because the excited blood is constantly moving out of the imaging region. Also, blood vessels often appear more narrow because signal from the slow-flowing blood at the edges of the vessels is difficult to detect. Signal is reduced by complex blood flow behavior, such as pulsatility, vorticity, and acceleration in tight turns, causing signal void due to dephasing.
In contrast, black blood angiography methods utilize a flow-related signal void. The magnetic resonance signals from flowing blood are suppressed, while the signals from stationary blood and tissue are optimized. In other words, flowing blood is made to appear dark or black on the magnetic resonance image due to an absence or minimum of resonance signal emanating from the blood. The black blood method is typically preferable to the white blood method because it is easier to make flowing blood appear dark for the aforementioned reasons. In addition, blood vessels on a black blood angiograph appear larger because the slow-moving blood at the edges is clearly imaged. Also, the black blood MRA provides more detailed depiction of small vessels where blood flow is slower.
In black blood MRA, the flow-related signal void can be generated by using spoiling gradients, pre-saturation RF pulses, or defocused flowing spins. The first two means are mostly used in field echo (FE) style sequences while the latter one is typically used in spin echo (SE) style sequences, such as fast spin echo (FSE) sequences.
In the past, proton density weighted (PDW) FSE sequences have been used to acquire black blood angiography images. For an n-echo FSE sequence (n=2, 4, . . . , 32, etc.) the first echo is oriented near the center of k-space, the second echo is located in the adjacent segment, and so on working out from the center.
In such an arrangement of k-space data, PDW images are acquired. These images typically exhibit good background tissue depiction. However, slow-flowing dipoles are refocused by the subsequent 180xc2x0 pulses contributing signal and resulting in xe2x80x9cfillingxe2x80x9d, i.e., black blood in the center of arteries and veins and white or gray blood along the blood vessel walls, in capillaries, and in areas with slower moving blood. The filling effect leads to falsified vessel definition. While this problem may be resolved by using pre-saturation RF pulses, this comes at the cost of increasing patient magnetic field dose (SAR) which is very critical on a high field system (xe2x89xa71.5 T).
Typically, black blood angiography images are processed using conventional minimum intensity projection algorithms. However, because minimum intensity projection algorithms enhance dark areas of signal void, it is difficult to distinguish dark vascular regions from dark background and/or cavities. In the past, the difficulty in distinguishing vascular and nonvascular dark regions has been resolved by using manually defined seed voxels, either vascular or nonvascular, as a starting point. From there, thresholding and connectivity criteria have been employed to selectively locate vascular signal voids. This image processing method is both slow and complicated, thus reducing ease and efficiency of black blood MRA applications. In addition, previous methods have not attempted to use two types of measurement, containing supplementary and complementary data, with different types of contrast.
The present invention contemplates a new method and apparatus which overcome the above-referenced problems and others.
In accordance with one aspect of the present invention, a method for generating a black blood magnetic resonance angiograph of a body portion includes exciting dipoles within a selected imaging region to produce magnetic resonance signals. A train of magnetic resonance echoes are induced after the excitation such that early echoes of the train are more heavily proton density weighted and later echoes are more heavily T2 weighted. The train of magnetic resonance echoes are phase and frequency encoded and received and demodulated into a series of data lines. The data lines are sorted between data lines from more heavily proton density weighted echoes and data lines from more heavily T2 weighted echoes. The more heavily proton density weighted data lines are reconstructed into a proton density weighted image representation, while the more heavily T2 weighted data lines are reconstructed into a T2 weighted image representation. The proton density weighted and T2 weighted image representations are combined to generated a combined image representation.
In accordance with a more limited aspect of the present invention, the combining step includes scaling the proton density weighted image representation and the T2 weighted image representation to a common maximum intensity level. The proton density weighted and T2 weighted image representations are averaged to form an averaged image representation. An edge enhanced image is computed from the T2 weighted image representation and the edge image representation is subtracted from the averaged image representation, forming an angiographic image representation.
In accordance with another aspect of the present invention, a magnetic resonance imaging system includes a magnet for generating a temporally constant magnetic field through an examination region. A radio frequency transmitter excites and inverts magnetic dipoles in the examination region to generate a train of magnetic resonance echoes. Gradient magnetic field coils and a gradient magnetic field controller generate at least phase and read magnetic field gradient pulses in orthogonal directions across the examination region. A receiver receives and demodulates the magnetic resonance echoes to produce a series of data lines. A sort processor sorts the data lines between proton density weighted data lines and T2 weighted data lines. An early echo volume memory stores proton density weighted data lines, while a late echo volume memory stores the T2 weighted data lines. An image processor reconstructs the proton density weighted data lines into a proton density weighted image representation and the T2 weighted data lines into a T2 weighted image representation. A combination processor combines the proton density weighted and T2 weighted image representations.
In accordance with a more limited aspect of the present invention, the combination processor includes a scaling processor which scales the proton density weighted and T2 weighted image representations to a common maximum intensity level. A processor combines the proton density weighted and T2 weighted image representations into a combined image representation. An edge image processor computes an edge image representation from the T2 weighted image representation. A processor combines the edge image representation and the combined image representation to form an edge enhanced image representation.
In accordance with another aspect of the present invention, a method for combining at least a first and second complex image representation having different preferential attributes is provided. The method includes scaling the first and second complex image representations to a common maximum intensity level. The first and second complex image representations are combined to form a combined image. A gradient image is calculated by differentiating at least one of the first and second complex image representations in order to enhance desired image features present in the at least one of the first and second image representations. Combining the gradient image with the combined image forms a desired feature enhanced image having enhanced supplementary and redundant information relative to the first and second complex image representations.
One advantage of the present-invention is that it is more scan time efficient.
Another advantage of the present invention is that it results in improved signal-to-noise ratio.
Another advantage of the present invention is that multiple contrast images, such as a proton density weighted image, a T2 weighted image, and a black blood angiogram, are acquired from a single scan.
Another advantage of the present invention is that it leads to more accurate vascular morphology and better depiction of hyperfine vessels.
Another advantage of the present invention is that it eliminates mis-registration error between proton density weighted images and T2 weighted images.
Yet another advantage of the present invention is that it reduces the SAR level.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.